To address the limitations of current Li-ion battery systems a significant amount of research has focused on the development of alternatives to the standard cathode and anode Li-ion intercalation materials. A number of high energy density Li-ion intermetallic anode materials (ex. Al, Si, Sn, Pb, Sb, Cu—Sn, Co—Sn, etc.) have the potential of providing two or more times the energy density of graphitic carbon. [R. A. Huggins, e.a., Journal of the Electrochemical Society, 1981. 128: p. 725] Intermetallic based anodes, particularly with nano-phase morphology, could potentially lead to major advances for Li-ion batteries in energy density, safety and cost. Despite their promise though, most all of the intermetallic anode systems suffer from excessive volumetric expansion, capacity loss and impedance growth as Lithium ions are cycled in and out of the materials. Significant advances have been made to mitigate these issues using both compositional and particle engineering strategies to the point that intermetallic anode materials are just beginning to show up in commercial, high volumetric energy density batteries serving smaller niche markets. In particular, Sony has introduced a Li-ion battery using a nano-particulate Sn—Co—Ti—C anode material (US2006/0121348A1). The material provides a significant volumetric capacity advantage over current graphite anode materials which is realized at the cell level. While a major advance, this material represents only a small step in utilizing the full potential of intermetallic systems. The success of this material has created another broad, industry wide effort to identify and develop intermetallic systems that can provide even greater capacity and performance advantages which will be critical to meet the demands of emerging applications such as EV's, PHEVs, e-Bikes, and UPS backup systems.
From a materials engineering standpoint, it has been clearly demonstrated that limiting the size of the intermetallic anode particles to the nano-scale and increasing the material disorder can significantly improve cycling reversibility, sometimes by an order of magnitude. This works by minimizing the build up of stress in the particles and reducing the impact of inherent particle expansion on material failure modes. It has also been recognized for some time that the most effective way to limit the volumetric expansion of an intermetallic material is to dilute the active component with an inactive phase. This approach avoids the need for lower voltage limits, which are difficult to design into a full cell, and shifts the balance of capacity to lower voltage, thus increasing the average cell voltage. Thus from a compositional standpoint, the question becomes what should the inactive “matrix” comprise to provide the greatest benefit in terms of overall material performance and value. Simple conductive additives like Cu and carbon have been used and in some cases specific intermetallic phases that can accommodate Lithium ions more effectively have been identified. Another approach that has demonstrated promise involves the formation of an insulating, Li-ion conductive phase within the intermetallic material, which along with mitigating the effects of volumetric expansion, improves lithium distribution within the particles and shields the lithitated active phase surface from ongoing reactions with the electrolyte, particle isolation and capacity loss. An early example of this was the development of the glassy Sn-oxide anode materials (Sn in a Li2O matrix) and more recently the tin phosphide based anode materials that form Li3P, an ion conductive phase, during discharge. While representing significant improvements in performance, these materials still have problems with irreversible capacity loss and cycle life and have not been commercialized. In the case of the phosphide systems, the ion conductive phase, Li3P, is not stable in the normal window of activity for tin based anodes, which limits its effectiveness.
A number of other strategies have been proposed and demonstrated to improve the performance of intermetallic anodes based on modifying the composition of simple, elemental intermetallic materials like Al, Sn, and Si. One example is Cu6Sn5(Fe) [K. D Kepler, J. T. V., M. M. Thackeray, Electrochemical and Solid State Letters, 1999. 2 (7): p. 307.]
The above approaches can solve the volumetric changes problem to some extent, however, the cycling performance is still to be improved at a high capacity.